1. Field of the Invention
The present invention relates generally to the field of digital radio signal communications. More particularly, the invention relates to a highly efficient broadcast burst for use in spatial diversity systems and that can be shared among different base stations.
2. Description of the Related Art
Mobile radio communications systems such as cellular voice radio systems typically have several base stations in different locations available for use by mobile remote terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a set of frequencies or channels to use for communications with the remote terminals. The channels are different from those of neighboring base stations in order to avoid interference between neighboring base stations. As a result, the remote terminals can easily distinguish the transmissions received from one base station from the signals received from another. In addition, each base station can act independently in allocating and using the channel resources assigned to it.
Such radio communications systems typically include a broadcast channel (BCH). The BCH is broadcast to all remote terminals whether they are registered on the network or not and informs the remote terminals about the network. In order to access the network, a remote terminal normally tunes to and listens to the BCH before accessing the network. A remote terminal will typically scan a range of likely frequencies when it wants to access the network until it finds the strongest or clearest BCH. It will then use the BCH signal for synchronization and use information in the BCH to request access to the network. Such a request typically results in an exchange of information about the network using separate control and access channels and ends in the remote terminal receiving an assignment to a particular base station.
Because the BCH is transmitted to all potential remote terminals within the range of a particular base station, it is typically broadcast omni-directionally or across a wide simultaneous directional range. This causes a great amount of interference and noise. It also consumes resources that might otherwise be used to carry traffic. Existing wireless systems employ a fixed reuse on the broadcast channel. The more resources that are used for traffic, the fewer resources there are left for traffic. In a system designed for transmission using spatial parameters, for example, phase and amplitude, across multiple elements of an antenna array, it is difficult to generate a wide area uniform signal from the array. Without a long term consistent signal, it is also difficult for a user terminal to measure frequency offset. If the signal contains any unpredictable phase changes in or between bursts the difficulty is increased.